In current power grid, when the system breaks down (e.g. single phase-to-ground fault), it is required for the controllable reactor functioning on the power grid to quickly increase the capacity to its nominal capacity and control the response speed within 100 ms with capacity not exceeding the nominal capacity of the controllable reactor so as to meet the requirement of inhibiting overvoltage and secondary arc current. However, current technology fails to satisfy such requirement (i.e. quick-response). At present, there are three ways for DC excitation system to obtain power: 1. external power source; 2. compensation winding; and 3. control-winding. If the controllable reactor does not have a reliable external power source, it is necessary to obtain power from itself. The disadvantage for self-feeding controllable reactor to obtain power through control-winding is that it will generate a large amount of harmonic wave, along with decline of the primary voltage of the rectifier unit with the increase of saturation level of the reactor, while by using compensation winding it can successfully control the amount of the harmonic wave, but it also has the problem with decline of the primary voltage.